Custom Exception Example in Java

Introduction

Custom Exception is a classic Java console program that demonstrates the concept with complete source code and sample output. Exceptions represent runtime errors; Java uses try-catch-finally to handle them safely.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Syntax

class <exception_name> extends Exception{  
    <exception_name>(String s){  
    super(s);  
    }  
}

Program

public class CustomException {
    static void validateInput(int number) throws InvalidInputException{
        if(number>100){
            throw new InvalidInputException("Exception");
        }else{
            System.out.println("The provided input is valid");
        }
    }
    
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter a number less than 100 :  ");
        int number = scanner.nextInt();
        try{
            validateInput(number);
        }catch(InvalidInputException e){
            System.out.println("Caught Exception - Input is greater than 100");
        }
    }
}

class InvalidInputException extends Exception{  
    InvalidInputException(String exceptionText){  
        super(exceptionText);  
    }    
}

Sample Output

Enter a number less than 100 :  
1000
Caught Exception - Input is greater than 100

When to use

Use this custom exception example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. The if statement runs the nested code only when the condition is true.

3. A println / print call writes text to the console — part of the sample output below.

4. A Scanner reads typed input from the keyboard (System.in).

5. A println / print call writes text to the console — part of the sample output below.

6. int number = scanner.nextInt(); updates a variable used in the calculation or output.

7. A println / print call writes text to the console — part of the sample output below.

8. Compare your console output with the sample output for Custom Exception to confirm the program behaves correctly.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.